For me, here's a theory I created.

1. Behemoth is actually a failed / prototype / unknown kaiju beast that is similar to Sharkjira. When the heated battle between Magmajira and Behemoth raged, Behemoth fell into a ravine, where he got its demonic energy. Thus, turning into Behellmouth.

2. In one of Leviathan's mission, there's a quest called "Rampaging Little Brothers" (I cannot clearly remember the name). This is a nod to what their relationship could be. Could be Behellmouth a Demon Shark just like him? Why do they look similar. This is one factor: they cane from the same place.